Voltage converters serve to convert an input voltage into an output voltage. In this case, input and output voltages can differ both in terms of their magnitude and in terms of their voltage type, that is to say DC voltage (DC—direct current) or AC voltage (AC—alternating current). Voltage converters in the context of the application should therefore be understood to mean, in particular, DC voltage converters, also designated hereinafter as DC/DC converters, and inverter bridges, also designated hereinafter as DC/AC converters. Inverters, which are used for example in a photovoltaic installation, a fuel cell system or for battery-fed standby power installations in a local power supply system, have as an output stage at least one DC/AC converter, upstream of which a DC/DC converter can be connected as an input stage. In an inverter, such a DC/DC converter connected upstream usually serves to increase the voltage variation range at the input of the inverter. By way of example, when an inverter is used in a photovoltaic installation, the input voltage present at the inverter changes if the operating point of photovoltaic modules of the photovoltaic installation is varied depending on the load and the insolation, i.e. the intensity of the solar irradiation.
In many cases of use, the voltage of a power source feeding a voltage converter is not constant. By way of example, it changes in a photovoltaic installation if the operating point of photovoltaic modules of the photovoltaic installation varies in an insolation- and load-dependent manner. In a battery-fed standby power installation, the battery voltage as input voltage of a voltage converter is dependent on the transmitted load and the charge state of the battery. The cell voltage of a fuel cell as input voltage of a voltage converter likewise varies to a special degree precisely in the low-load range. An additional factor is that in many cases one would like to be able to connect different types of PV modules, fuel cells or batteries to a voltage converter, such that a largest possible input voltage range of the converter enables a greatest possible flexibility.
Voltage converters usually do not continuously load the power source connected to their input. This is caused for example by a non-continuous power flow in the case of an AC current that is output, or else a pulsed switching of power semiconductor switches in the voltage converter. In order to minimize voltage dips occurring at the power source on account of the current being drawn in a non-continuous manner, and thus to enable a high average power drawn from the power source, a capacitor arrangement is usually provided at the input of a voltage converter. In the simplest case, the capacitor arrangement consists of a capacitor connected in parallel with the input of the voltage converter. If such a capacitor arrangement is positioned between two voltage converters in the case of an arrangement comprising a plurality of voltage converters, it is usually designated as an intermediate circuit capacitor arrangement. In the context of this application, independently of the position of the capacitor arrangement, in a generalizing manner hereinafter the circuit in which the capacitor arrangement is arranged is designated as intermediate circuit, following the usual terminology. The capacitance of the capacitor arrangement is chosen depending on the magnitude of the maximum voltage dips that can be afforded tolerance during the the non-continuous current drawing (voltage ripple), operating parameters, the topology of the voltage converter and, in particular, the minimum voltage for maximum energy conversion. In the case of inverters in the field of photovoltaics, input or intermediate circuit capacitors having a capacitance of a few millifarads are not unusual for the voltage converters. At the same time, the voltages applied to the capacitor arrangements are in the range of 1000 volts or more. Capacitors having such a capacitance and dielectric strength are large and heavy. Moreover, they are expensive and, under certain circumstances, available only to an insufficient extent. If, for a desired capacitance, capacitors having the required dielectric strength are not obtainable, it is known to connect two capacitors having lower dielectric strength in series. Ideally, half of the voltage which is present at the series circuit formed by the two capacitors is then present in each case at the two capacitors. On account of component tolerances, however, different capacitors differ in terms of their properties, for example with regard to their internal resistance and their leakage current. As a result, different voltages can be established at the two series-connected capacitors during operation, which reduces the voltage range that can be utilized overall. For matching the voltages, it is known to bring the center tap between the two capacitors, via a resistance bridge or via a bridge having active switching elements, to a potential lying exactly between the potentials at the connections of the capacitor arrangement. In the case of the series circuit formed by the capacitors, however, the total capacitance of the capacitor arrangement decreases to a value of only half the capacitance of one of the capacitors.